You can install it from the retail discs via this method.
Log in to Steam and click on Library.
Right-click on the game, select Delete local content, and confirm.
Insert the first disc into your computer.
Close Steam (Steam > Exit).
Press Win + R to open Run
In the Run window type:
"C:\Program Files\Steam\Steam.exe" -install E:
Replace E: with the CD/DVD drive you are installing from if is not correct.
Replace C:\Program Files\Steam if your Steam installation is not in the default location.
- Press OK. Steam will launch and ask you to sign in if you do not have your password saved. Your installation should continue from the disc.
It's the official Steam method of installing a game from retail discs that would normally install via Steam, so it works for any game in this category, including many Call of Duty games, most Valve games, and other Steam-enabled games like Civilization 5.
However, note that there have been several patches to Modern Warfare 3 since launch, which you will have to download and install via Steam before you can play the game online.
Playing online is also likely to require a fast internet connection and a decent amount of bandwidth towards your cap, so keep that in mind if want to do anything mulitplayer with it.